Field devices, in particular field devices which are used together with sensors to measure fill levels or limit levels, are often based on delay measurements. In delay measurements, the signal delays of radar signals or guided microwave pulses are determined. In general, the delay of an electromagnetic wave is measured. The desired measurement value, for example a fill level or limit level, is subsequently determined from these signal delays.
The signals are of a particular frequency. The radar signals and the microwave signals can be allocated to the high-frequency technology (HF technology) range. As signals which are in the high-frequency range, signals in the frequency range of up to 2 GHz are generally used as guided microwave signals, and signals in the range of from 5 GHz to 79 GHz and above are used as radar signals.
For safety reasons, it may be necessary for the electronics of the field device to be separated from the measurement environment, for example an inside of a container filled with a filling medium, in an explosion-protected manner. The separation consists for example of a gas-tight seal. This can prevent explosive substances or gas mixtures from the container interior reaching the electronics of the field device and igniting there. The IEC (International Electrotechnical Commission) standard IEC 60079-1:2007 is identical to the standard for explosive atmospheres, ÖVE-ÖNORM EN 60079-1, and relates to equipment protection by pressure-resistant enclosures “d” (Equipment protection by flame proof explosures “d” or enclosures “d”). Equipment which complies with explosion protection class “d”, known as Exd equipment, meet the particular requirements for the construction and testing of electrical equipment in the pressure-resistant enclosure “d” type of ignition protection, which is intended to be used in regions at risk of gas explosions.
EP 2 093 846 A1 discloses a gas-tight conductor feed-through for a field device, which can provide explosion protection. The conductor feed-through is designed to be coaxial and is used for example in a frequency range of between 5 and 28 GHz.
EP 2 683 022 A1 describes a gas-tight hollow conductor coupling for coupling an electromagnetic transmission signal from a high-frequency module into a hollow conductor. The hollow conductor coupling may comprise a round disc made of a circuit board substrate, which has a metal-coated edge for soldered connection to the hollow conductor.
EP 2 683 023 A1 describes a hollow conductor coupling comprising a hollow conductor, the internal diameter of which widens towards a planar radiator element.
Antennae may be protected by means of a process separation and/or by means of a filling which covers the antenna opening and protects it from penetration by foreign substances. However, despite antennae being enclosed or being filled in part, it is possible that moisture may form in the hollow conductor.